This invention relates to tunnel support linings and a method of creating such tunnel linings using flat plywood sheets bent and installed so that the horizontal edges abut and are secured in place in a prestressed condition.
Underground tunnels and shafts, such as the type used for roadways and railways, sewer control tunnels, subways and similar structures, require a temporary or primary supporting lining when driven through almost any strata except fault-free hard rock. The lining is needed for retaining the strata in which the tunnel is formed and maintaining the integrity of the tunnel against crumbling until a finished concrete liner can be installed, usually after the excavated tunnel is completed.
The prior art for constructing such temporary tunnel linings includes the practice of using circular metal ribs which retain wooden boards in a relationship parallel to the axis of the tunnel, known as rib and lagging. Other examples of tunnel linings include preformed arcuate concrete lining segments which join to form a circle in the tunnel, such as shown in U.S. Pat. No. 4,397,583.
It is known in the art to construct a tunnel lining from either arcuate lining segments or a plurality of relatively narrow sheet metal panels formed together to conform to the side of the wall of the tunnel, such as shown in U.S. Pat. No. 4,464,084.
It is desirable that the tunnel lining requires a minimum of effort for construction within the tunnel and at low cost as to both material and labor.
It is also important to provide a lining which can be installed immediately after the tunnel has been cut directly behind the shield or tunnel boring machine. In some rock conditions, a tunnel may be self-supporting as long as the tunnel surface contains a sufficient amount of moisture, but when the rock dries out, the rock will crumble and the surface erode.
It is also an advantage to provide a method of lining which can be adapted to the conditions of the tunnel as it is excavated. Although test borings are taken of the rock before the tunnel is bored, it is never clear exactly what conditions will be on the exposed surface of the tunnel walls. Some sections of a tunnel may be harder or softer rock. Thus, some sections of the tunnel may have an irregular or uneven surface where hard inclusions have been removed during excavation.